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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) | 12 Months Ended | 0 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2014 State | Dec. 31, 2013 | Dec. 31, 2012 | Aug. 10, 2014 | Feb. 13, 2014 | Aug. 09, 2013 | Feb. 13, 2015
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of insurance companies | 13 mcy_NumberOfSubsidiaries |  |  |  |  |  |
Number of States in which Entity Operates | 13 us-gaap_NumberOfStatesInWhichEntityOperates |  |  |  |  |  |
Percentage of direct premiums written as private passenger automobile lines of insurance | 77.00%mcy_PercentageOfDirectPremiumsWrittenAsPrivatePassengerAutomobileLinesOfInsurance |  |  |  |  |  |
Percentage of private passenger automobile premiums written in California | 83.00%mcy_PercentageOfPrivatePassengerAutomobilePremiumsWrittenInCalifornia | 81.00%mcy_PercentageOfPrivatePassengerAutomobilePremiumsWrittenInCalifornia | 78.00%mcy_PercentageOfPrivatePassengerAutomobilePremiumsWrittenInCalifornia |  |  |  |
Percentage by which dividend income on non redeemable preferred stock, partnership, common stock is partially tax-sheltered | 70.00%mcy_PercentageByWhichDividendIncomeOnNonRedeemablePreferredStockPartnershipCommonStockIsPartiallyTaxSheltered |  |  |  |  |  |
Other liabilities | $ 207,028,000 us-gaap_OtherLiabilities | $ 170,665,000 us-gaap_OtherLiabilities |  |  |  |  |
Marketable securities, fixed maturities | 0 us-gaap_MarketableSecuritiesFixedMaturities |  |  |  |  |  |
Marketable securities, equity securities | 11,700,000 us-gaap_MarketableSecuritiesEquitySecurities |  |  |  |  |  |
Insurance companies security deposits | 18,000,000 mcy_SecurityDepositsByInsuranceCompanies | 17,000,000 mcy_SecurityDepositsByInsuranceCompanies |  |  |  |  |
Deferred policy acquisition cost amortization | 526,208,000 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense | 505,517,000 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense | 477,788,000 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense |  |  |  |
Advertising expenses | 23,000,000 us-gaap_AdvertisingExpense | 20,000,000 us-gaap_AdvertisingExpense | 19,000,000 us-gaap_AdvertisingExpense |  |  |  |
Impairment charges | 0 us-gaap_AssetImpairmentCharges | 0 us-gaap_AssetImpairmentCharges | 0 us-gaap_AssetImpairmentCharges |  |  |  |
Goodwill impairment loss | 0 us-gaap_GoodwillImpairmentLoss | 0 us-gaap_GoodwillImpairmentLoss |  |  |  |  |
Premiums written, net | 2,840,000,000 us-gaap_PremiumsWrittenNet | 2,730,000,000 us-gaap_PremiumsWrittenNet | 2,650,000,000 us-gaap_PremiumsWrittenNet |  |  |  |
Credit facility amount | 150,000,000 us-gaap_LineOfCreditFacilityAmountOutstanding |  |  |  |  |  |
FFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Notional amount | 114,000,000 invest_DerivativeNotionalAmount / dei_LegalEntityAxis = mcy_FFLMember |  |  |  |  |  |
AFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Notional amount | 144,000,000 invest_DerivativeNotionalAmount / dei_LegalEntityAxis = mcy_AFLMember |  |  |  |  |  |
Options Granted Prior To 2008 [Member] | Minimum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Option vesting term, years | 4 years |  |  |  |  |  |
Options Granted Prior To 2008 [Member] | Maximum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Option vesting term, years | 5 years |  |  |  |  |  |
Options Granted Subsequent To January 1, 2008 [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Option vesting term, years | 4 years |  |  |  |  |  |
Building [Member] | Minimum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Useful life, in years | 30 years |  |  |  |  |  |
Building [Member] | Maximum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Useful life, in years | 40 years |  |  |  |  |  |
Furniture Equipment And Purchase Software [Member] | Minimum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Useful life, in years | 3 years |  |  |  |  |  |
Furniture Equipment And Purchase Software [Member] | Maximum [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Useful life, in years | 7 years |  |  |  |  |  |
Software [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Useful life, in years | 7 years |  |  |  |  |  |
Swap [Member] | FFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Term of swap agreement |  |  |  | 3 years |  |  |
Notional amount | 114,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_FFLMember |  |  |  |  |  |
Swap agreement collateral | 30,000,000 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_FFLMember |  |  |  |  |  |
Swap [Member] | AFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Term of swap agreement |  | 3 years |  |  |  |  |
Notional amount | 144,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_AFLMember |  |  |  |  |  |
Swap agreement collateral | 40,000,000 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_AFLMember |  |  |  |  |  |
LIBOR [Member] | Swap [Member] | FFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  | 1.40%us-gaap_DerivativeBasisSpreadOnVariableRate / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_FFLMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
LIBOR [Member] | Swap [Member] | AFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  | 1.20%us-gaap_DerivativeBasisSpreadOnVariableRate / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_AFLMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |
Secured Note No1 [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Debt instrument, fair value | 120,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = mcy_SecuredNoteNo1Member |  |  |  |  |  |
Secured Note No2 [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Debt instrument, fair value | 20,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = mcy_SecuredNoteNo2Member |  |  |  |  |  |
Call Option [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Other liabilities | 200,000 us-gaap_OtherLiabilities / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_CallOptionMember | 100,000 us-gaap_OtherLiabilities / us-gaap_OptionIndexedToIssuersEquityTypeAxis = us-gaap_CallOptionMember |  |  |  |  |
Unsecured Debt [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Debt instrument, fair value | 150,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_UnsecuredDebtMember | 50,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_UnsecuredDebtMember |  |  |  |  |
Revolving Credit Facility [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Credit facility amount | $ 150,000,000 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |  |  |
Scenario, Forecast [Member] | LIBOR [Member] | Swap [Member] | FFL [Member] |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  |  |  |  |  | 1.35%us-gaap_DerivativeBasisSpreadOnVariableRate / us-gaap_DerivativeByNatureAxis = us-gaap_SwapMember / dei_LegalEntityAxis = mcy_FFLMember / us-gaap_StatementScenarioAxis = us-gaap_ScenarioForecastMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember

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